The present invention relates generally to converting electrical signals from one form to another, and more particularly to signal conversion using coarse and fine digital to analog converters.
Digital to analog converters (xe2x80x9cDACxe2x80x9ds) have been used in positioning systems for many years. Generally, a digital controller, such as a microprocessor, a digital signal processor (xe2x80x9cDSPxe2x80x9d), or dedicated digital circuitry, generates control commands for commanding some type of actuator to a desired position. Generally, the control commands from the digital controller are digital electrical signals. Because many actuators require commands in the form of analog electrical signals, DACs are used to convert the digital signals into analog voltage or current signals.
Generally, many positioning systems are required to rapidly move an actuator from one position to a new position, and then accurately hold the actuator at the new position or within a small range of that position. In addition, the actuator may move over a large range relative to the resolution of the positioning system (e.g., the position change may represent many bits worth of movement compared to the movement equal to one bit in the digital command). This generally requires that the digital commands contain many bits so that the actuator may be positioned to within a small tolerance at a commanded position over a large range of movement.
As an example, the read (and/or write) heads used in data devices generally require movement over a large range (e.g., seek mode), and accurate positioning at a particular position (e.g., track mode). Examples include the optical heads used in optical disk drives, such as compact disc (xe2x80x9cCDxe2x80x9d) and digital video disc (xe2x80x9cDVDxe2x80x9d) drives, and the magnetic heads used in hard drives.
As another example, micro-electromechanical (xe2x80x9cMEMxe2x80x9d) devices may require high resolution positioning and a large movement range relative to the bit resolution. One specific example is a MEM movable mirror assembly, as described in Laor et al., U.S. Pat. No. 6,295,154, issued Sep. 25, 2001, entitled OPTICAL SWITCHING APPARATUS,xe2x80x9d commonly assigned herewith and incorporated herein by reference. As described in detail in Laor et al., a micromirror generally is rotatable about two axis and is driven magnetically using some combination of permanent magnets and electromagnetic coils. The micromirror positioning is precisely controlled by electrical signals sent to the electromagnetic coils. Because analog signals are used to control the coils, the mirror""s position is generally continuously variable over its range of motion. The precise positioning of the micromirror is accomplished by way of calibration and feedback, so that the positioning system is able to sense the mirror""s position and make corrections.
A digital controller such as a DSP may be used to provide digital control commands to a micromirror. The DSP may send a coil current command in digital form to a DAC, which converts the command to an analog signal. The analog signal may then be sent to a coil current amplifier, which drives the coil and moves the micromirror. Generally, with respect to the DAC, high resolution is preferred in order to control the commanded position very precisely and with very little error due to quantization and noise. Generally, the DAC is required to have a wide voltage range to support a wide range of positions and rapid movement between them, as well as high resolution and low noise to provide accurate tracking.
In the prior art, a large dynamic range DAC with high resolution has been used in the above applications. One disadvantage, however, with a large dynamic range DAC is that it is typically expensive compared to other DACs. For example, a 16-bit quad DAC component may cost more than twice as much as a 12-bit octal DAC component, making a single 16-bit DAC more than four times as expensive as a single 12-bit DAC.
Another potential disadvantage with a large dynamic range DAC is that it may have a large differential non-linearity (xe2x80x9cDNLxe2x80x9d) in its high resolution setting. For example, a commercially available 16-bit DAC may only be guaranteed monotonic to 14 or 15 bits. This may cause positioning discontinuities when a digital control command is converted to analog and sent to an actuator.
Another potential disadvantage with some prior art DACs, and in particular low bit-count or low resolution DACs, is that they may not be designed with high quality analog sections, and thus may inject high frequency noise into their analog output voltages. This noise may feed through the current driver and cause undesirable actuator motion during position tracking operations.
One prior art method of reducing the noise on the DAC output is to add a filter. However, one problem with using a filter is that it adds phase delay to the control current and slows down the transient response. Generally, a filter providing sufficient noise attenuation may introduce so much phase delay that it may even make the position control loop unstable or uncontrollable.
A preferred embodiment of the present invention comprises an analog micromirror apparatus having two lower resolution DACs instead of one high resolution DAC. One DAC may function as a coarse DAC and the other DAC may function as a fine DAC. Generally, the output of the coarse DAC has a higher gain than the output of the fine DAC so that it may support wide voltage swings and provide the coarse positioning portion of the position control signal. The fine DAC has lower gain so that it may support accurate tracking control and provide the fine positioning portion of the control signal. The voltage outputs of the two DACs may be combined in an analog summing junction to form the complete analog control command.
Another preferred embodiment of the present invention comprises a coarse DAC, a fine DAC, and a low pass filter connected to the output of the coarse DAC. The low pass filter generally prevents high frequency noise with the high gain of the coarse DAC signal from transferring through to the combined analog signal. The filter may also be switchable to filter or not filter the coarse DAC output. The filter may be switched off during large voltage changes to provide rapid response, and switched on during for small voltage changes to attenuate noise from the coarse DAC. The filter may comprise a capacitor, which may be pre-charged during large movements when the filter is switched off. Generally, the filter capacitor is thus already charged when the filter is switched on for a tracking operation, reducing the switch transient and avoiding a glitch on the voltage output to the actuator.
In accordance with a preferred embodiment of the present invention, an analog micromirror apparatus comprises a digital controller generating a digital control command, a coarse DAC having an input coupled to the digital controller and having a coarse DAC output, a fine DAC having an input coupled to the digital controller and having a fine DAC output, a summing amplifier coupled to the outputs of the coarse DAC and the fine DAC, and having a summed analog output, the summed output representative of the digital control command, and a driver element for orienting a mirror element, the driver element coupled to the analog output of the summing amplifier.
In accordance with another preferred embodiment of the present invention, a digital to analog converter apparatus comprises a coarse input for receiving a coarse digital value, a fine input for receiving a fine digital value, wherein the combined coarse and fine digital values represent a digital signal, a coarse DAC having an input coupled to the coarse input, and having an analog coarse output, a fine DAC having an input coupled to the fine input, and having an analog fine output, a filter coupled to the analog coarse output of the coarse DAC, and a summing amplifier having a first input coupled to the analog coarse output and a second input coupled to the analog fine output, and having a summed analog output providing an analog signal.
In accordance with another preferred embodiment of the present invention, a position control circuit for a positioning system comprises a digital controller, wherein the digital controller generates a digital control command and apportions the digital control command into a digital coarse value and a digital fine value, a coarse DAC having an input coupled to the digital controller for receiving the digital coarse value, and having an analog coarse output, a fine DAC having an input coupled to the digital controller for receiving the digital fine value, and having an analog fine output, a switch having a first input coupled to the analog coarse output, a filter having a capacitor coupled to an output of the switch, and a summing amplifier having a first input coupled to the analog coarse output and a second input coupled to the analog fine output, and having a summed analog output, corresponding to the digital control command, for controlling an actuator in the positioning system.
In accordance with another preferred embodiment of the present invention, a method for providing control commands to a micromirror device comprises generating a digital control command, apportioning the digital control command into a digital coarse value and a digital fine value, converting the digital coarse value into an analog coarse signal, converting the digital fine value into an analog fine signal, summing the analog coarse and fine signals, in accordance with a coarse/fine gain ratio, to generate an analog position signal corresponding to the digital control command, and providing the analog position signal to a driver element to orient the micromirror device.
In accordance with another preferred embodiment of the present invention, a method of generating an analog signal from a digital signal comprises apportioning the digital signal into a digital coarse value and a digital fine value, converting the digital fine value into an analog fine value, converting the digital coarse value into an analog coarse value, summing the analog coarse and fine values, in accordance with a coarse/fine gain ratio, to generate the analog signal corresponding to the digital signal, and switching on a filter coupled to the analog coarse value after a time period during which the analog coarse value remains constant.
An advantage of a preferred embodiment of the present invention is that the two narrow dynamic range DACs and their support circuitry costs less than a single wide range DAC.
Another advantage of a preferred embodiment of the present invention is that it has a lower DNL than a high resolution DAC. For example, two 12-bit DACs used to form a coarse/fine pair with a 16:1 gain ratio may effectively have 16-bit monotonicity. Generally, each 12-bit DAC has 12 bit monotonicity, and the fine DAC only contributes about {fraction (1/16)}th to the total DNL, improving the effective DNL to be monotonic to 16 bits.
A further advantage of a preferred embodiment of the present invention is that it provides true 16 bit resolution over its 16 bit range. Generally, a resistor string architecture is used in a 12 bit DAC, which provides accurate voltage taps to generate the output from the DAC. With this type of architecture in a DAC, less than one bit monotonicity may be achieved.
Another advantage of a preferred embodiment of the present invention is that the filter reduces noise without introducing phase delay. Generally, the filter is disabled during large-scale voltage changes, allowing for rapid voltage stewing without phase loss due to filtering. During stationary positioning or tracking mode, the filter is switched into the coarse DAC path, attenuating the noise from the coarse DAC. The fine DAC responds to small disturbances without any loss of phase because the filter is not connected to it. In addition, the coarse DAC filter generally does not introduce phase delay during tracking mode because the coarse DAC output is not changing.
Yet another advantage of a preferred embodiment of the present invention is that pre-charging the filter capacitor generally reduces glitches caused by switching the filter into the circuit.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.